The present invention relates generally to special effects generators for use in combining multiple input video signals into a single video output signal, and more particularly, to such a generator that uses digital logic circuitry to generate the special effects.
Special effects generators are used in television studios to combine several video signals into a single composite video signal. A typical special effects generator combines a portion of one signal with portions of one or more other signals under the control of one or more control levers. A typical special effect is a "horizontal wipe" in which portions of the images are represented by two input signals are combined into a single output image with a vertical line separating the two image portions. The horizontal position of the vertical line is determined by the position of a manually actuated lever. The line can be made to proceed horizontally across the screen in response to movement of the lever to provide a "wiping" effect where one image portion is gradually displaced by the other image portion. Other similar special effects are "vertical wipes" in which a horizontal line separating the image portions can be moved vertically across the screen, and "corner wipes" where a portion of one video signal is displayed in one corner of the output image and a portion of another video signal comprises the rest of the output image. The corner image is generally rectangular in shape, and has a size that is controllable by the position of a control lever. The size of the corner image can be made to increase in size in response to movement of the control lever until the corner image comprises the entire output image. Another special effect is a "QUAD effect" wherein portions of four video images can be combined into a single output image with each of the four image portions being displayed in a separate quadrant of the output image. The relative sizes of the four images comprising the output image can be varied by actuating a control lever which adjusts the position of the horizontal and vertical lines separating the four images.
Special effects generators may conveniently be classified into two categories: analog and digital. Analog special effects generators utilize analog signal processing circuitry including horizontal and vertical sawtooth and parabolic generators that generate analog potentials proportional to the horizontal and vertical positions of a scanning spot. Analog comparators are used for comparing various combinations of these analog signals with internally generated control signals proportional to the position of a lever or other control. Switching signals are generated as a result of the analog comparisons for operating electronic switches that apply portions of various video signals to a transmission medium in accordance with the result of the comparison.
Analog special effects generators are generally difficult to align and suffer from other drawbacks including time and temperature drift and susceptibility to noise. Consequently, digital special effects generators have been developed that eliminate most of the problems associated with analog special effects generators. One such digital special effects generator is described in U.S. Pat. No. 3,821,468 issued to Albert E. Busch on June 28, 1974 and assigned to the same assignee as the assignee of the present invention. The above Busch patent is incorporated herein by reference.
The apparatus described in the Busch patent provides a wide variety of special effects including diamonds, circles, horizontal and vertical wipes and others. Consequently, the circuitry required to provide these effects results in a rather complex and costly system. It has been found that for many applications, some of the special effects provided by the generator described in the Busch patent, such as the circle which require complex circuitry to implement, are not necessary. It has also been found that other effects that are relatively easy to implement and not provided by the generator described in the Busch patent are desirable. Such effects include the corner wipe and the QUAD effect described above. Furthermore, new circuit developments have made it possible to simplify the circuitry of the prior art generators while simultaneously improving performance.